Bhagavad Gita 6.25

शनैः शनैरुपरमेद् बुद्ध्या धृतिगृहीतया।आत्मसंस्थं मनः कृत्वा न किञ्चिदपि चिन्तयेत् ॥६.२५॥

śanaiḥ śanairuparamed buddhyā dhṛtigṛhītayāātmasaṃsthaṃ manaḥ kṛtvā na kiñcidapi cintayet

Little by little, let him attain steadiness of the intellect by holding it firmly; having made the mind establish itself in the Self, let him not think of anything else.

गीता